The evolution of NGC 7027 at radio frequencies: A new determination of the distance and core mass

We present the results of a 25 yr program to monitor the radio flux evolution of the planetary nebula NGC 7027. We find significant evolution of the spectral flux densities. The flux density at 1465 MHz, where the nebula is optically thick, is increasing at a rate of 0.251% ± 0.015% yr -1, caused by the expansion of the ionized nebula. At frequencies where the emission is optically thin, the spectral flux density is changing at a rate of -0.145% ± 0.005% yr-1, caused by a decrease in the number of ionizing photons coming from the central star. A distance of 980 ± 100 pc is derived. By fitting interpolated models of post-AGB evolution to the observed changes, we find that over the 25 yr monitoring period, the stellar temperature has increased by 3900 ± 900 K and the stellar bolometric luminosity has decreased by 1.75% ± 0.5%. We derive a distance-independent stellar mass of 0.655 ± 0.010 M⊙ adopting the Blöcker stellar evolution models, or about 0.04 M ⊙ higher when using models of Vassiliadis & Wood. A Cloudy photoionization model is used to fit all epochs at all frequencies simultaneously. The differences between the radio flux density predictions and the observed values show some time-independent residuals of typically 1%. A possible explanation is inaccuracies in the radio flux scale of Baars and coworkers. We propose an adjustment to the flux density scale of the primary radio flux calibrator 3C 286, based on the Cloudy model of NGC 7027. We also calculate precise flux densities for NGC 7027 for all standard continuum bands used at the VLA, as well as for some new 30 GHz experiments. © 2008. The American Astronomical Society. All rights reserved.